Ion source for providing a supply of charged particles having a controlled kinetic energy distribution

ABSTRACT

An ion source device that includes apparatus for pulse injecting ions into a field-free drift region where those ions separate according to their velocities or kinetic energies. A cyclically varying electric field is established at the end of the fieldfree drift region. Each charged particle enters the electric field at a time determined by its velocity and interacts with the field in a manner dependent upon the phase of the field upon receipt of that particle. The cyclically varying electric field decreases the velocity distribution for particles received during the first half of a cycle, and increases the distribution for particles received during the second half of a cycle. Mass analyzing apparatus is disposed to receive and measure the concentrations of species of charged particles expelled from the cyclically varying field. The output provided by the mass analyzing apparatus also facilitates variation of the cyclically varying electric field to provide a particular species with a desired velocity or kinetic energy distribution.

Wiley et al.

[ [ON SOURCE FOR PROVIDING A SUPPLY OF CHARGED PARTICLES HAVING A CONTROLLED KINETIC ENERGY DlS'llRlBUTlON [75] Inventors: William C. Wiley, Conshohocken,

Pa.; John P. Carrico, Royal Oak, Mich.

[73] Assignee: The Bendix Corporation, Southfield,

Mich.

[22] Filed: Feb. 28, 1972 [21] Appl. No.: 229,816

[52] US. Cl. 250/292, 250/294 [51] Int. Cl. l-l0lj 39/34 [58] Field of Search. 250/41.9 G, 41.9 DS, 41.9 TE

[56] References Cited UNITED STATES PATENTS 2,790,080 4/1957 Wells 250/4l.9 TF

3,187,180 6/1965 Welldig.... 250/4l.9 ME

3,582,648 6/1971 Anderson 250/4l.9 DS

3,621,240 11/1971 Cohen 250/41.9 DS

7 2; I... Tum/r525 i l Primary ExaminerJames W. Lawrence Assistant Examiner-C. E. Church Attorney, Agent, or FirmJohn S. Bell [57] ABSTRACT An ion source device that includes apparatus for pulse injecting ions into a field-free drift region where those ions separate according to their velocities or kinetic energies. A cyclically varying electric field is established at the end of the field-free drift region. Each charged particle enters the electric field at a time determined by its velocity and interacts with the field in a manner dependent upon the phase of the field upon receipt of that particle. The cyclically varying electric field decreases the velocity distribution for particles received during the first half of a cycle, and increases the distribution for particles received during the second half of a cycle. Mass analyzing apparatus is disposed to receive and measure the concentrations of species of charged particles expelled from the cyclically varying field. The output provided by the mass analyzing apparatus also facilitates variation of the cyclically varying electric field to provide a particular species with a desired velocity or kinetic energy distribution.

2 Claims, 3 Drawing Figures D/SPL/I/ 0 PAIENTEnmnzmsu FIG] o/spmy O VAR/ABLE 1 04 77/66 FIGZ F/ELD FIG. 3

SOURC E 3.0 VELOCITY OUT VELOCITY IN ION SOURCE FOR PROVIDING A SUPPLY OF CHARGED PARTICLES HAVING A CONTROLLED KINETIC ENERGY DISTRIBUTION BACKGROUND OF THE INVENTION 1. Field of the Invention Ion source devices for providing supplies of charged particles to devices such as mass spectrometers.

2. Brief Description of the Prior Art One known ion source device removes charged particles having undesirable energies from a sample in order to reduce the kinetic energy differences between charged particles of a species and thereby increase the resolution of a mass spectrometer measuring the concentrations of various species of charged particles. The term species of charged particles is used herein to identify particles having the same mass to charge ratio. The primary drawback of this device is that the removal of charged particles from a sample reduces the concentration or strength of the signal to be measured by a mass spectrometer and thereby decreases the sensitivity of the apparatus.

One known device does not provide a monoenergetic supply of ions in order to overcome the ambiguities introduced into spectrometer measurements by energy differences between various ions of a species. Instead, this device utilizes a radial electrostatic field and a uniform magnetic field to separate various charged particles so that ions of one species having different energies follow different paths through the electric and magnetic fields to be focused on a detector, while ions of different species follow paths such that they do not reach the detector. The primary drawback of this apparatus is that it is extremely difficult to adjust both the electric and magnetic fields so that they will cooperate with each other to separate and refocus charged particles of one species to a predetermined position and to simultaneously separate the charged particles of that one species from particles of all other species. A slight misadjustment of either .one of the fields will cause the detector to receive either more than one species, or only a portion of one species.

In photoelectron spectrometry, the accuracy of measurements of the concentration of charged particles as a function of their kinetic energy can be increased by magnifying the kinetic energy differences between charged particles of a species. In this art, electrons are photoemitted from a surface, and the energy distribution of the emitted photoelectrons is measured in order to determine the composition of the surface. The energy spread of emitted photoelectrons is typically too small to be compatible with the resolution of measuring apparatus. The prior art does not teach apparatus that can be used effectively to either reduce or magnify the kinetic energy distribution of charged particles.

SUMMARY OF THE INVENTION The device of this invention includes apparatus that provides a cyclically varying electric field for altering the kinetic energies or velocities of various charged particles to provide a species of charged particles with a predetermined kinetic energy distribution. The term kinetic energy distribution of a species is used herein to refer to the difference between the kinetic energies of the various particlesof that species. The kinetic energy distribution of a species of particles having identical masses is unambiguously related, or in other words equivalent to the velocity distribution of that species. These two terms will, therefore, be used interchangeably herein. The terms ions and charged particles" will also be used interchangeably herein to designate any electrically charged particles including electrons. The term phase is used herein as it is customarily used in electronics to identify cyclic variation. The phase of a field at any one instant is a value expressed in degrees indicating the position in a cycle of the field at that instant. The field interacts with each received charged particle and alters the kinetic energy or velocity of the particle in a manner determined by the phase of the field upon reception of the particle.

The velocity distribution of a species of ions can be increased by injecting the ions into the electric field during the first half of a cycle, with the high energy ions being injected into the field before the lower energy ions. If desired, the apparatus of this invention can be used to reduce the kinetic energy distribution of a species of charged particles to provide a substantially mono-energetic supply of charged particles. In addition, the velocity distribution of a species of ions can be increased with the apparatus of this invention by injecting those ions into the cyclically varying electric field during the second half of a cycle, with the high kinetic energy or velocity ions entering the field before the lower energy ions.

In the embodiment illustrated herein, a variable voltage source for providing an electric signal having a sinusoidally varying AC component and a monopole electrode structure which receives that electric signal are used to provide a cyclically varying electric field for interacting with charged particles. The apparatus for injecting charged particles having different kinetic energies into the cyclically varying field at different times includes apparatus for ionizing particles and for injecting the ionized particles into a field-free drift region separating the ionizing apparatus and the cyclically varying electric field. The ions separate according to their velocities or kinetic energies as they travel through the drift region toward the electric field, so that the high velocity particles enter the field before those with lower velocities. The manner in which the field interacts with and alters the velocity distribution of received ions is determined by the phase of the field upon receipt of various ions and the strength of the field. The phase or portion of a cycle during which a selected species of ions enter the electric field can be controlled by controlling the timing of the injecting apparatus, the length of the field-free drift region, the ion drift energy or energy of ions upon entering the drift region, and the frequency of the cyclically varying electric field. By appropriate control of these parameters and the strength of the cyclically varying electric field, the apparatus of this invention can be used to provide any one species of charged particles selected from a wide range of different species with any one of a wide range of energy distributions including a monoenergetic energy distribution in which all particles of a selected species have substantially the same energy. The device of this invention also provides a very strong signal, or in other words a high concentration of particles of the species of interest since it alters the energies or velocities of particles instead of eliminating particles from a sample in order to provide a desired energy distribution.

The cyclically varying electric field provided by the apparatus illustrated herein interacts with and expels received charged particles. Mass analyzing apparatus is illustrated for receiving these expelled particles, separating the various species of expelled charged particles according to their mass-to-charge ratio, and measuring the concentration of at least a portion of particles of a species. A sequence of different concentration measurements is obtained by periodically injecting identical samples into the electric field and varying the mass analyzing apparatus. The change in the concentration measurements as the analyzing apparatus is varied identifies the velocity distribution of a species of charged particles. The velocity distribution is manifested as an ion detector output signal comprising a mass spectral peak having a peak width proportional to the velocity distribution. Since the mass peak output provided by the mass analyzing apparatus for the series of samples identifies the velocity distribution provided by a cyclically varying electric field having a particular value, the value of the cyclically varying field can be rapidly and conveniently adjusted either automatically or manually to provide a species with any desired velocity distribution.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features, and advantages of this invention, which is defined by the appended claims, will become apparent from a consideration of the following description and the accompanying drawings in which:

FIG. I is a partially perspective, schematic illustration of a device for providing a supply of charged particles having a controlled velocity distribution. The device includes apparatus for providing a cyclically varying electric field, apparatus for injecting charged particles into that field, a drift region, and apparatus for analyzing the concentrations of charged particles expelled from the electric field;

FIG. 2 is a graph that illustrates the manner in which the velocity of a particle depends on the phase of the electric field provided by the apparatus of FIG. 1 upon receipt of that particle; and FIG. 3 is a graph of the output signal provided by the analyzing apparatus of the device of FIG. I as a function of the variation of that analyzing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a device that includes apparatus 12 for providing a cyclically varying electric field, apparatus 14 for injecting charged particles into that field, and mass analyzing apparatus 16 for analyzing the concentrations of species of charged particles expelled from the electric field. The apparatus 12 for providing a cyclically varying electric field includes a monopole structure comprising a rod shaped conducting electrode 18 having a surface 20 that may be either cylindrical or hyperbolic in cross section, and a V-shaped or trough shaped conductive element 22 spaced from rod 18. Two openings 24 and 26 are formed in conductive element 22 to allow charged particles to enter and leave the region 27 between rod 18 and element 22. A voltage source 28 for providing a cyclically varying electric signal is connected across rod 18 and the V- shaped trough 22. When an electric signal having an oscillating component, such as an electric signal having a DC or direct current component and a sinusoidally varying AC component is applied across rod 38 and trough 22, a dynamic potential distribution of the form (12 (y,z,t) =f(t) Hz 0 (y Z is created between those two elements, where:

x, y, z are spatial coordinates along the directions indicated in FIG. I;

Z is the z axis length of the dynamic field provided in the region bounded by rod 18 and V-shaped element 22;

f(t) is a sinusoidally varying electric signal provided by source 28 and having the form V, V,,,. sin 11!, where: V,,,. is the magnitude of a DC signal component of the signal provided by source 28;

V,,,. is the amplitude of an AC signal component of the signal provided by source 28;

v is the frequency of the V signal component; and

t is time.

Voltage source 28 is a variable source for providing electric signals of different frequencies, AC amplitudes and DC magnitudes to rod 18. Variation of the signals provided permits the device 10 to be used with charged particles having different mass-to-charge ratios and permits alteration of the time interval over which a sample of particles is injected into the electric field.

The apparatus 14 for injecting charged particles into the cyclically varying electric field includes an electron emitting filament 30, an electron control grid 32, an electrostatic focusing lens 33, an anode 34, backing plate 36, and an ion control grid 38. These elements define an ionizing region 40. A tube or cylinder 42, which is grounded and electrically insulated from ion control grid 38 separates the injecting apparatus 14 from the field providing apparatus 12 and provides a field-free drift region between those apparatuses. Ion control grid 38 includes a mesh section 44 so that ions can flow from ionizing region 40 into the field-free drift region defined by tube 42. A gating logic circuit 46 controls the transmission of voltage pulses from source 28 to control grids 32 and 38 to thereby control the timing of the operation of those grids.

The apparatus 16 for analyzing charged particles expelled from the field provided by the apparatus 12 includes an electromagnet 48 having two pole pieces 50 and 52. An electromagnet power supply 54 is connected to electromagnet 48. When electromagnet 48 receives a signal from supply 54, an analyzing magnetic field that spatially separates ions according to their mass-to-charge ratio is created in region 56 between pole pieces 50 and 52. A detector 58 is disposed downstream from the analyzing magnetic field to receive charged particles and provide an output electric signal proportional to the concentration of those received particles. A measuring and display apparatus 60 receives signals from detector 58 and provides an output indicating the concentration of received charged particles. A plate 62 having a slit 64 formed therein is placed to control the entry of particles into the magnetic field, and a similar plate 66 having a slit 68 formed therein is placed between the magnetic field and detector 58 in order to control the flow of charged particles to that detector. The ionizing region 40, cyclically varying electric field region 27, and mass analyzing region 56 are enclosed in an evacuative metal housing 70 that includes a valve 72 through which particles can be introduced into the housing.

In operation, a sample of particles to be ionized is introduced through valve 72 into region 40. A voltage pulse is transmitted from source 28 through gating logic 46 to control grid 32 to establish a potential on that grid that permits electrons to flow from filament 30 through ionizing region 40 to anode 34. This electron flow ionizes the particles in region 40. Filament 30, control grid 32, lens 33, anode 34, backing plate 36, and control grid 38 are voltage biased with respect to V-shaped electrode 22. This voltage bias determines the kinetic energy of the ions in the drift region defined by tube 42. Different ions of a species in region 40 will possess different kinetic energies or velocities. A voltage pulse is then supplied to control grid 38 to draw ions out of region 40 into the field-free drift region defined by tube 42. The ions traverse this field-free drift region toward the cyclically varying electric field pro vided by structure 12, and separate according to their kinetic energies. The high kinetic energy or high velocity ions of a species enter the cyclically varying electric field before those having lower velocities or kinetic energies. The cyclically varying electric field interacts with each ion in a manner dependent at least in part on the phase of the field at the time that the ion enters the field. The ion drift energy or energy imparted to ions drawn into the field-free drift region, the timing of the voltage pulse supplied to grid 38, the length of drift tube 42, and the frequency of the cyclically varying electric field provided bystructure 12 are, therefore, all controlled so that the particles of a selected species being analyzed enter the cyclically varying electric field during a predetermined portion of a cycle and are thus provided with a preselected velocity distribution.

The graph of FIG. 2 illustrates the particular velocity variation provided to ions of one species as a function of the phase of the cyclically varying electric field upon receipt of an ion. Graphs for other species will have a form similar to that of FIG. 2, but may have a slightly different slope. As can be seen from FIG. 2, the velocity of an ion expelled from the cyclically varying field relative to the velocity of that ion entering the field is lower for ions entering the field during the early portion of the first half of a cycle than it is for ions entering the field later in the first half of a cycle. That is, ions entering the field during the early part of the first half of the cycle are decelerated or accelerated only slightly. Those entering the field somewhat later in the first half of a cycle are given larger accelerations. The

velocity or kinetic energy distribution of a species of ions can, therefore, be reduced by injecting ions of that species into the electric field during the first half of a cycle.

Conversely, the velocity distribution of a species of ions can be increased by injecting the ions of that species into the field during the second half of a cycle. Again, higher kinetic energy or velocity ions of a species reach the field before the lower energy ions of that species. However, the velocity of an ion leaving the field relative to the velocity of that ion entering the field is greater for ions entering the electric field during the early part of the second half of a cycle than it is for ions entering the field later in the second half of a cycle. Thus, high energy or velocity particles enter the field early in the second half of a cycle and are accelerated. Particles entering somewhat later in the second half of a cycle are accelerated to a lesser extent. And,

particles entering near the end of the second half of a cycle are decelerated.

The particular values of the operating voltages supplied to the various elements of the device 10 are related to the dimensions of that embodiment. Both dimensions and operating signal values may be varied over a wide range in different embodiments. However, in one effective and typical embodiment, rod 18 has a diameter of approximately 0.6 cms and a length of approximately 13 cms. The separation between rod 18 and trough 22 is approximately 0.4 cms. In this embodiment, the voltage supplied from source 28 in order to establish an electric field between rod 18 and trough 22 has a DC component with a magnitude on the order of several hundred volts, an AC voltage with an amplitude on the order of several thousand volts or less, and a frequency on the order of several MHZ. Drift tube 42 has a length of approximately 5 cms, and the voltage bias signals applied to the elements of injecting apparatus 14 are selected so that ions entering tube 42 possess drift energies on the order of 1,400 electron volts. With these dimensions and operating signal values, all ions having masses less than 28 AMU enter the cyclically varying electric field in less than one-half of a cycle. The relationship between the various dimensions and operating voltages of this embodiment satisfy the equation:

A t +1 M/2E- 10 7/2 where:

At, is the time interval during which ions are drawn into drift tube 42;

l is the length of tube 42;

M is the mass measured in AMU of the ions of the selected species being measured;

E is the drift energy measured in electron volts of the selected species of ions;

1- is the period of the cyclically varying electric field.

This equation can be used in the design of other embodiments.

The velocity distribution provided to a particular species of charged particles also depends in part on the amplitude of the electric field provided by the apparatus 12. For example, a field having one amplitude will cause a particular species of charged particles injected into the field during the first half of a cycle to have a smaller velocity distribution than will a field having a different amplitude receiving the same species of charged particles during a similar portion of a cycle.

An output signal can be obtained from mass analyzing apparatus 16 that facilitates amplitude adjustment of the electric field to provide a particular species of charged particles with a desired velocity distribution. This output identifies both the velocity distribution and the intensity or concentration of a species of particles and is referred to by those in the art as a mass peak. The peak width of a mass peak for a species of particles depends on the velocity distribution of that species. A mass peak is obtained by injecting subsequent identical samples of ions into the electric field provided by the apparatus 12 and adjusting the value of the analyzing magnetic field so that the magnetic field has a different value upon receipt of subsequent identical samples. Slits 64 and 68 are sufficiently narrow so that only a portion of the charged particles of one species of any one sample will be permitted to reach detector 58. The

graph of FIG. 3 illustrates the intensities or concentrations of subsequent signals received by detector 58 as the strength of the analyzing magnetic field is varied. Portion 74 of the graph of FIG. 3 comprises a mass peak identifying the concentrations of one species reaching the detector 58 in subsequent tests. The amplitude of the electric field provided by the apparatus i2 is held constant while performing the series of tests required to provide a mass peak for a particular species. However, the slope of a mass peak for a particular species obtained in a series of tests performed with an electric field having one value will be different from the slope of the mass peak obtained for the same species and a series of tests performed with the electric field having a different amplitude. Adjusting of the amplitude of the electric field provided by apparatus 12 to increase the slope of a mass peak for a species of charged particles decreases the velocity distribution that the particles of that species will have as they leave the magnetic field.

Having thus described one embodiment of this invention, a number of modifications will readily occur to those skilled in the art. For example, cyclically varying electric fields other than the particular fields illustrated herein may be used to alter the kinetic energy distributions of species of charged particles. Electrode structures other than the monopole electrode structure may be used to provide cyclically varying electric fields for altering velocity distributions. The electromagnet power supply 54- can be synchronized to voltage source 28 so that the device can be operated in an automatic scan mode to automatically provide an output comprising mass peaks for a wide range of different species that may be included in a sample. And, the analyzing apparatus 16 which spatially separates various species of charged particles can be replaced by apparatus responsive to the temporal separation between various species of charged particles.

What is claimed is:

i. A device for generating a supply of charged particles having a predetermined kinetic energy comprising:

means responsive to an input signal for generating groups of charged particles having a predetermined initial kinetic energy distribution and accelerating said groups of charged particles in a predetermined direction;

means receiving said accelerated groups of charged particles for generating a field free drift region wherein the charged particles within each of said groups spatially separate along said predetermined direction in accordance with their kinetic energy;

means having three mutually perpendicular axes for receiving said spatially separated groups of charged particles and for generating a dynamic electric field cyclically varying along at least one of said mutually perpendicular axes wherein the potential distribution d; of said dynamic field varies as a function of the square of the distance along said at least one axis, said dynamic electric field having at least two phases, a first phase wherein the potential distribution of the field increases as a function of time and a second phase wherein the potential distribution of the field decreases as a function of time, said electric field operative to reduce the kinetic energy distribution of the separated charged particles received during said first phase dependent upon the time the charged particles within each group enters the field, said dynamic electric field further operative to cause the received charged particles to oscillate for a predeterminable number of cyclical variations further reducing the kinetic energy distribution of the charged particles before being ejected from the electric field;

means for generating a cyclical electrical voltage having a predetermined frequency, a predetermined DC component, and a predetermined AC component, said cyclical voltage applied to said electric field generating means determines the potential distribution and phase of said dynamic electric field; and

means responsive to the cyclical electrical voltage for generating the input signal having a predetermined phase relationship to said cyclical voltage, said input signal communicated to said means for accel erating operative to cause said groups of accelerated charged particles, after passing through said field free region, to enter said dynamic field during said first phase.

2. The device as claimed in claim 1 wherein said means for generating said dynamic electric field is a monopole structure comprising:

a V-shaped conductor having two planar surfaces angularly disposed with respect to each other and joined along a common edge, said V-shaped conductor further having an entrance aperture and an exit aperture disposed along said commonly joined edge and spatially separated a predetermined distance from each other; and

a conductive rod symmetrically disposed between said planar surfaces and spaced therefrom;

said monopole structure further having a z-axis along a line normal to the axis of said rod, passing through the center of said rod and the commonly joined edge of said V-shaped conductor, and a yaxis disposed normal to the plane formed by said z axis and the axis of said rod, further wherein the potential distribution is given by the equation (y z, ac ac Sin /zo (Y 2 where:

y and z are said y and z axes about which said dynamic field cyclically varies;

z is a distance along the z axis in the region bounded by the surface of said rod and said commonly joined edge;

t time;

V the DC component of said cyclical voltage;

V the AC component of said cyclical voltage; and

v the frequency of said cyclical voltage. 

1. A device for generating a supply of charged particles having a predetermined kinetic energy comprising: means responsive to an input signal for generating groups of charged particles having a predetermined initial kinetic energy distribution and accelerating said groups of charged particles in a predetermined direction; means receiving said accelerated groups of charged particles for generating a field free drift region wherein the charged particles within each of said groups spatially separate along said predetermined direction in accordance with their kinetic energy; means having three mutually perpendicular axes for receiving said spatially separated groups of charged particles and for generating a dynamic electric field cyclically varying along at least one of said mutually perpendicular axes wherein the potential distribution phi of said dynamic field varies as a function of the square of the distance along said at least one axis, said dynamic electric field having at least two phases, a first phase wherein the potential distribution of the field increases as a function of time and a second phase wherein the potential distribution of the field decreases as a function of time, said electric field operative to reduce the kinetic energy distribution of the separated charged particles received during said first phase dependent upon the time the charged particles within each group enters the field, said dynamic electric field further operative to cause the received charged particles to oscillate for a predeterminable number of cyclical variations further reducing the kinetic energy distribution of the charged particles before being ejected from the electric field; means for generating a cyclical electrical voltage having a predetermined frequency, a predetermined DC component, and a predetermined AC component, said cyclical voltage applied to said electric field generating means determines the potential distribution and phase of said dynamic electric field; and means responsive to the cyclical electrical voltage for generating the input signal having a predetermined phase relationship to said cyclical voltage, said input signal communicated to said means for accelerating operative to cause said groups of accelerated charged particles, after passing through said field free region, to enter said dynamic field during said first phase.
 2. The device as claimed in claim 1 wherein said means for generating said dynamic electric field is a monopole structure comprising: a V-shaped conductor having two planar surfaces angularly disposed with respect to each other and joined along a common edge, said V-shaped conductor further having an entrance aperture and an exit aperture disposed along said commonly joined edge and spatially separated a predetermined distance from each other; and a conductive rod symmetrically disposed between said planar surfaces and spaced therefrom; said monopole structure further having a z-axis along a line normal to the axis of said rod, passing through the center of said rod and the commonly joined edge of said V-shaped conductor, and a y-axis disposed normal to the plane formed by said z axis and the axis of said rod, further wherein the potential distribution is given by the equation phi (y,z,t) (Vdc + Vac sin Nu t) 1/zo2 (y2 - z2) where: y and Z are said y and z axes about which said dynamic field cyclically varies; zo is a distance along the z axis in the region bounded by the surface of said rod and said commonly joined edge; t time; Vdc the DC component of said cyclical voltage; Vac the AC component of said cyclical voltage; and Nu the frequency of said cyclical voltage. 